1. Field of the Invention
This invention relates to an image-forming apparatus that transfers a developer to a recording medium to form an image, and more particularly to an image-forming apparatus that forms images with a charged non-magnetic single-component developer
2. Description of the Related Art
Image-forming apparatuses are known in the prior art which comprise an electrostatic latent image carrier such as a photosensitive drum on the surface of which electrostatic latent images are formed, development means that convey a developer such as a toner to the surface of the electrostatic latent image carrier and thus develop the electrostatic latent images, supply means for supplying the developer to the development means, and layer thickness restricting means for restricting the thickness of the developer layer adhering to the surface of the development means before the developer faces to the electrostatic latent image carrier. With this type of image-forming apparatus, the developer is conveyed to the surface of the electrostatic latent image carrier by the supply means and development means, the electrostatic latent images are developed by that developer, and the images are then formed by transferring the developer to a recording medium. The thickness of the developer layer is restricted by the layer thickness restricting means before it faces to the electrostatic latent image carrier, so that images can be formed in a uniform density.
As the developer noted above, two types are known, namely one being a two-component developer system containing a toner and a carrier, and the other being a single-component developer system containing no carrier. In recent years, in the interest of maintaining image-forming apparatuses with ease and making them more compact, instead of the two-component systems, single-component developer systems have come into wide use. With single-component developer systems, a substance corresponding to the carrier is contained in the toner, and, when the toner is moved using magnetic force, the toner is made to contain magnetic materials. However, the magnetic materials are opaque, and hence especially with color development, it is more suitable to use a non-magnetic developer. That being so, in recent years, it is becoming increasingly common to use a non-magnetic single-component developer in electrophotographic image-forming apparatuses that form images by transferring a developer such as toner to a recording medium.
A non-magnetic single-component developer is charged either positively or negatively, and develops the electrostatic latent images using electrostatic attraction. The known methods for charging the toner or other non-magnetic single-component developers include a method wherein a toner is rubbed together by, e.g., rollers (in some cases functioning as both of the supply means and development means noted earlier), thereby friction-charging the toner, and a method wherein layer thickness restricting means is made of an electrically conductive material, and the toner is charged by injecting electrical charges into the toner via the layer thickness restricting means.
With either of the methods noted above, however, it is very difficult to charge the toner uniformly and stably, as discussed below. Accordingly, good images cannot be formed stably with conventional image-forming apparatuses.
More specifically, a toner that adheres to the surface of the development means in a layer the thickness of which is restricted is not all available for development; a portion thereof remains adhering to the surface of the development means. Development means are often constituted with, e.g., rollers. In the former of the two methods that is based on friction-charging, the toner that remains adhering to the development means is again subjected to friction-charging. When that happens, the amount of friction-charging becomes excessive in some of the toner, so that it adheres strongly to the development means and sometimes hinders the charging of other toner. As a consequence, adequate friction-charging is not effected in the toner, so that not only the image density declines, but inter-toner friction increases, and in some cases friction-charging of a polarity opposite to the desired charging polarity occurs. This results in so-called inverse fogging wherein toner adheres to the background in electrostatic latent images on the electrostatic latent image carrier.
With a non-magnetic single-component developer, in particular, charging speed is slow compared to the case of a two-component developer system, and when forming images in a low-temperature, low-humidity environment, fogging readily occurs, immediately after replenishing the toner with new toner, or when forming images after the toner has deteriorated. In addition, even worse fogging tends to occur in areas connecting to black solid printed areas where much toner is consumed.
In order to resolve these problems, the latter of the two methods, namely the method based on electrical charge injection, has been conceived. With this latter method, however, charging efficiency is highly dependent on the resistance of the toner, and so is readily affected by the environment, such that it is very difficult to control the amount of charging in the developer. If the toner resistance is made sufficiently small or innovations are made in the additives, it may be possible to reduce occurrences of fogging, but, depending on the environment, there is the possibility of transfer faults occurring or of a decline in fine line reproducibility.